Mr. H Falls from the Top of a Climbing Wall

A 65 Kg Mr. H fell 8.0 m from the top of a climbing wall and he is going 12.52 m/s until he hits the mats. If the mats are 40 cm thick, how much force did they exert on him to prevent him from hitting the floor?

I think you need to use the vf^2=vi^2 + 2a(df-di) formula to find the acceleration. Then use that in the Fnet= ma but the problem is eluding me.

Since he falls at constant speed, 12.52 m/s, until he hits the mat, vi= 12.52. Since the mat prevents him from hitting hte floor, vf must be 0. di is 0 because he is just hitting the mat, df must be no larger than 40 cm= .4 m if he does not hit the floor.

(I am taking d= 0 at the top of the mat. You could as well take di= 40 and df= 0. That will change only the sign of the acceleration.)